Transistors formed over a glass substrate or the like have been manufactured using amorphous silicon, polycrystalline silicon, or the like, as typically seen in liquid crystal display devices. Although transistors manufactured using amorphous silicon have low field-effect mobility, they have an advantage of being able to use larger glass substrates. Further, although transistors manufactured using polycrystalline silicon have high field-effect mobility, they have a disadvantage of not being suitable for large glass substrates.
In contrast to the transistors manufactured using silicon, a technique in which a transistor is manufactured using an oxide semiconductor and applied to an electronic device or an optical device has attracted attention. For example, Patent Document 1 and Patent Document 2 disclose a technique in which a transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor and such a transistor is used as a switching element or the like in a pixel of a display device.